Wideband Code Division Multiple Access (WCDMA) is a widely deployed wireless communication network access protocol. Among other applications, WCDMA is the primary air interface for the 3G Universal Mobile Telecommunications System (UMTS) technology.
Interference Cancellation is a technology being considered for uplink WCDMA to improve the performance in mixed data and voice scenarios. In interference cancellation, a high-data-rate signal is detected first. After detection, the high-data-rate signal is cancelled from the received signal, prior to the detection of low-data-rate signals such as voice. This general approach is described in U.S. Pat. No. 7,389,099, “Method and apparatus for canceling interference from high power, high data rate signals,” the disclosure of which is incorporated herein by reference in its entirety.
The basic operations of interference cancellation consist of three steps. First, data bits carried in the interfering signal are detected. Next, the interference signal is regenerated at the receiver. Finally, the regenerated interfering signal is cancelled from the received signal.
The first step can be done before decoding (pre-decoding interference cancellation) or after decoding (post-decoding interference cancellation). The second step is basically to mimic how the transmitted bits arrive at the receiver. This involves going through the operations performed at the transmitter (what the transmitter has done to the data bits) and channel filtering (what the channel has done to the data bits).
Pre-decoding interference cancellation is preferable due to its lower latency. When an interfering signal uses 10 msec Transmission Time Interval (TTI) and the desired latency is on the order of a few slots, then the transport format information can not be detected via the reception of Enhanced Dedicated Physical Control Channel (E-DPCCH). This is because for 10 ms TTI (i.e. 15 WCDMA slots), an E-DPCCH message is also spread over 10 ms, which means that E-DPCCH can only be received very reliably after 10 ms. Thus, within the desired latency (a few slots) of pre-decoding interference cancellation, the transport format cannot be detected reliably through the reception of E-DPCCH. Early E-DPCCH detection is possible provided that E-DPCCH is boosted in power. However, the boosting E-DPCCH feature is not available for 10 msec TTI.
Accordingly, to perform pre-decoding interference cancellation for 10 msec TTI, the transport format (e.g., modulation, spreading factor, and number of spreading codes) must be blindly detected.
Blind spreading factor detection is known in the art, and is addressed in the IS-95 standard. For example, one known solution is to run a Viterbi decoder once for each rate (or spreading factor) hypothesis and formulate decision metrics based on the decoder outputs, as described in U.S. Pat. No. 5,566,206, “Method and apparatus for determining data rate of transmitted variable rate data in a communications receiver,” incorporated herein by reference in its entirety. However, these techniques have latency of 10 msec, and thus are not suitable for the case of 10 msec TTI.
An optimal pre-decoding blind spreading factor detector is described by E. Davis, et al., in the paper titled, “A MAP blind bit-rate detector for variable-gain multiple-access systems,” published in the IEEE Trans. Commun., vol. 51, pp. 880-884, June 2003, which is incorporated herein by reference in its entirety. Another optimal pre-decoding blind spreading factor detector, and additionally a suboptimal scheme based on the autocorrelations of partially despread values, is described by M. Juntti and K. Pajukoski in the paper titled, “Blind spreading factor detection for DS-CDMA,” published in the Proc. IEEE Int. Symp. Personal, Indoor, Mobile Radio Commun., pp. 1395-1399, London, U. K., September 2000, which is incorporated herein by reference in its entirety. The optimal pre-decoding blind spreading factor detection schemes described in these papers are unnecessarily complex due to their treatment about the amplitude of the desired signal term as a function of the spreading factor hypothesis. Additionally, the papers do not elaborate how to obtain key parameters necessary for the optimal detectors.